rts: Don't rely on EXTERN_INLINE for slop-zeroing logic

Previously we relied on calling EXTERN_INLINE functions defined in
ClosureMacros.h from Cmm to zero slop. However, as far as I can tell,
this is no longer safe to do in C99 as EXTERN_INLINE definitions may be emitted
in each compilation unit.

Fix this by explicitly declaring a new set of non-inline functions in
ZeroSlop.c which can be called from Cmm and marking the ClosureMacros.h
definitions as INLINE_HEADER.

In the future we should try to eliminate EXTERN_INLINE.